The present invention relates to a noise filter, in particular, to a noise filter which controls the occurrence of distortion of a digital signal waveform and functions effectively in a digital signal circuit.
Conventional noise filters for use with digital signals were constructed with inductance elements and capacitor elements being combined in such a form as T-type or .pi.-type and the like.
As one example, the T-type is exemplified in FIG. 12, in which the inductance elements L.sub.1 and L.sub.2 mentioned above are inserted in series with the input/output line 14 between the input terminal 11 and the output terminal 12, and also a capacitor element C.sub.1 is inserted in parallel between the point 15 connecting both inductance elements L.sub.1 and L.sub.2 and a ground terminal 13.
But, the conventional noise filter constructed as such, when used in high-speed digital signal circuit, causes distortion of the digital signal waveform concurrent with the elimination of noise, that is, causing the occurrence of overshoot, undershoot and ringing, and an increase in the peak value thereof as compared with the original waveform, thus resulting in malfunctions and the like of a circuit. It is assumed that this arises from such various reasons as the inevitable mismatch in impedance between said noise filter and signal system before or after thereof, thus causing a reflected wave to be produced.
For example, the conventional noise filter described above was connected between TTL circuits 2 and 3 as noise filter 4 as shown in FIG. 13, and the waveform was observed. Even though the original digital signal S from the TTL circuit 2 showed a clear waveform as in FIG. 14, the waveforms of the input signal S.sub.1 and output signal S.sub.0 of the noise filter 4 were significantly distorted. This can be seen in FIGS. 15 and 16. .DELTA.V in FIG. 16 shows increment in peak value with respect to the original digital signal S.
Usually when distortion of a waveform due to ringing and the like is to be controlled, if the equivalent circuit in FIG. 12 is taken by way of example, a resistance is connected in series between the inductance elements L.sub.1 and l.sub.2 and between the input terminal and output terminal.
But, if resistance is connected in series to the inductances L.sub.1 and L.sub.2, the waveform of the signal becomes smaller to that extent with the result of malfunctioning of a circuit.